Lignocellulosic biomass refers to plant biomass that is composed of cellulose, hemicellulose, and lignin. This biomass comes in many different types, which may be grouped into four main categories: (1) wood residues (including sawmill and paper mill discards), (2) municipal paper waste, (3) agricultural residues (including corn stover and sugarcane bagasse), and (4) dedicated energy crops (which are mostly composed of fast growing tall, woody grasses). In all these categories the carbohydrate polymers (cellulose and hemicelluloses) are tightly bound to the lignin, by hydrogen and covalent bonds.
Fermentation of lignocellulosic biomass to ethanol and butanol is an attractive route to process energy feedstocks that supplement the depleting stores of fossil fuels. Biomass is a carbon-neutral source of energy, since it comes from dead plants, which means that the combustion of ethanol produced from lignocelluloses will produce no net carbon dioxide in the earth's atmosphere. Also, biomass is readily available, and the fermentation of lignocelluloses provides an attractive way to dispose of many industrial and agricultural waste products. Finally, lignocellulosic biomass is a renewable resource. Many of the dedicated energy crops can provide high energy biomass, which may be harvested multiple times each year.
One barrier to the production of ethanol from biomass is that a large fraction of the sugars necessary for fermentation present in the form of lignocellulose. Lignocellulose has evolved to resist degradation and to confer hydrolytic stability and structural robustness to the cell walls of the plants. This robustness or “recalcitrance” is attributable to the crosslinking between the polysaccharides (cellulose and hemicellulose) and the lignin via ester and ether linkages, thus creating a material that is physically hard to access. This means that for an efficient use of the components from the lignocellulose, said lignocellulose should be disintegrated and/or separated and/or decrystallized, to allow enzymes to be able to contact the cellulose and hemicellulose for conversion into oligo- and monosaccharides, which then in turn can be used for many purposes, e.g. for bio-ethanol formation and further derivation.
One of the most commonly used methods for degradation of the lignocellulose is heating of the wet biomass in the presence of an acid. Two major problems occur with such a treatment: 1) the heating may only be short, because otherwise too many unwanted byproducts are formed from the carbohydrates; and 2) it is difficult to produce a biomass slurry with more than 30% w/w solids, which is necessary for an economic use in further processing.
Accordingly there is still need for an efficient process in which lignocellulose is degraded to disclose the components thereof for further processing.